In the past, to perform servotrack writing in computer disk drives, the readwrite head position in the drive has typically been controlled by a single positioning servo loop using a mechanically engaging push-pin on an external push-pin arm driven by an external VCM. These systems have a number of disadvantages, particularly including mechanical play between the thru-hole and the push-pin.
Alternately, dual servo systems employing both a master and a slave servo system have come into use. In these the slave servo system optically locks the arm of the disk drive bearing the read-write head into a synchronous relationship with a reference arm of the servotrack writing apparatus, and precise measurement of the position of this reference arm is used during the actual writing of servotracks into the media surfaces of the disk drive. In essence, this technique is that taught in U.S. Pat. No. 5,315,372, by one of the present inventors.
Unfortunately, as desired servotrack densities have become very high, say exceeding 8,000 tracks per inch, techniques used for optically locking the drive arm and the reference arm into the synchronous relationship needed has not been able to match the possible resolution of the reference arm position measuring system, which typically uses very precise laser interferometric methods. Thus the highest possible accuracy of servotrack writing has not been achieved.
Further, many reference arm position measuring laser interferometer systems use only a single-beam single-pass type system, and thus are susceptible to repeating and non-repeating master arm pivot eccentricity errors.
Therefore, to progress beyond micro-meter resolution and into the nanometer resolution range in servotrack writing systems, what is needed is improved slave servo system photo sensor sensitivity. Further desirable is concurrent reduction of master servo system eccentricity errors in such servotrack writing systems. The present invention addresses both of these needs, and thereby derives the full potential of servotrack writing systems which employ it.